The best introduction is probably the Campaigns of Alexander the Great installment of the Osprey Military Campaign series. It's focused concisely on the military campaigns, a bit less on the other biograpnical aspects. This short book is full of battle and campaign maps as well as detailed illustrations of the different soldiers and weapons used: hypaspists, phalanx, Persian Immortals and Macedonian Companions, the sarissa, Alexander's cavalry, etc. The best full formal biography, IMO, is the one by Peter Green, which I believe is the standard modern text. Also, of course the documentary In Search of Alexander and the book based on that BBC series.
I have the Napoleon book from the Osprey Military Campaign series. They do a great job just delivering the facts. This one includes lots of dispatches from Napoleon that are as entertaining as informational. He tells his brother, while trying to not loose Spain, do what I tell you and wewill win, do what you want and you will loose.
He should have said instead: "Do what you want, and I will replace you before we lose!" But that's a good quote. All Ospreys are a great introduction to generals, campaigns and battles. I read one of their battle books, this one on Hannibal's incredible triumph at the Battle of Cannae against the Romans. These books really come into their own because their graphic-intense format allows readers to clearly visualize what they're reading in the text. Also, there's no discursive fat in these books: they deliver what you're looking for and what you need to know about a specific subject.
Oh yeah, Osprey! I have a bunch of their books regarding the Napoleonic Wars, all being in the Campaign series, and thus not very long. Thanks for the suggestion! I'll have to pick it up in my next order.